1. Field of the Invention
The present invention relates to an asynchronous read channel shaped toward generalized partial response characteristics.
2. Description of the Related Art
Magnetic tape cartridges include magnetic tape to store data to be saved and read back at a subsequent time. A magnetic tape drive writes the data to magnetic tape, typically as a set of parallel tracks, and subsequently a magnetic tape drive reads back the data. To read back the data, a magnetic tape drive typically comprises parallel read heads to read each of the parallel tracks, a drive system for moving a magnetic tape with respect to the read heads such that the read heads may detect magnetic signals on the magnetic tape, and a read channel for digitally sampling magnetic signals detected by the read heads and providing digital samples of the magnetic signals. The digital samples are then decoded into data bits, and the data bits from the parallel tracks are combined into the data that was saved. The read channel typically requires an equalizer for each of the read heads to compensate for the change in the signal due to the magnetic recording properties of the write head, the magnetic tape, and the read head. Magnetic tapes may be interchanged between tape drives, such that a magnetic tape written on one tape drive will be read by another tape drive. Variation in the response of the read heads to the variously written magnetic tapes may result in unacceptably poor read back of the recorded signals.
In order to achieve higher cartridge capacities and improved performance, advances in several technical areas are necessary. A real density increase, i.e. increase in linear and/or track density, is key to achieving higher storage capacities. Increasing a real density decreases the distance between adjacent bit cells leading to an increase in intersymbol-interference (ISI). Higher track density also implies narrower track width, narrower write/read heads and closer head spacing, leading to losses in signal-to-noise ratio (SNR). Also issues of intertrack-interference become critical. Signal equalization and sequence detection at high linear and track densities requires optimal control of ISI. Adaptive equalization on user data is also very important in order to guarantee best read-channel performance during tape operation and to mitigate variations in the recording channel transfer characteristics.
Read channels for magnetic storage systems may be designed according to one of two basic architectures, an asynchronous architecture and a synchronous architecture. In the synchronous architecture, the analog to digital converter (ADC) is driven by a variable frequency oscillator (VFO) that is usually controlled by a digital timing-recovery unit such that the readback signal is sampled synchronously with respect to the write clock. The synchronous signal samples are first equalized and then provided to the detection circuit. Typically, timing information is extracted from the equalized sample values. Synchronous architectures are not typically used in tape systems.
In read channels having an asynchronous architecture, the ADC converter is driven by a fixed clock with rate 1/T′ and the sampling of the readback signal is done asynchronously with respect to the write clock. The synchronization of the signal samples is accomplished digitally using interpolative timing recovery (ITR).
In asynchronous tape drive systems, the read signals are typically shaped towards a partial-response target characteristic. For example, the (1−D2) Class IV Partial Response (PR4) or the (1+D−D2−D3) Extended PR4 (EPR4) polynomials can be used as partial response targets. When implementing maximum-likelihood detection, read channels that employ partial-response signal shaping are referred to as PRML channels.
More general targets for signal shaping are specified by generalized partial response characteristics of the type (1+g1 D+g2 D2+ . . . gN DN), where the coefficients gi, i=1, . . . , N can assume non-integer values. This approach also allows one to incorporate noise prediction within the equalization/detection function in order to whiten the noise process at the input of the detector and also minimize its energy. Disk drive systems use noise-predictive maximum-likelihood (NPML) detectors to detect a readback signal shaped toward a generalized partial response polynomial target.